Does this require clients of your server to all support HTML so that they can screen-scrape partials to understand the results of their operations, or does your server support two output formats?
Two output formats is a reasonable solution, but one that you don't have to maintain if you go with a framework (such as Angular) that can render JSON back into HTML. It really boils down to whether you want to support a more complicated server or a more complicated client (or drop support for any clients that aren't yours).
It's a traditional web app, so it works with HTML. We don't plan to have anything else any time soon so the priority was on the time to have the HTML ready to go.
I did a Rails app serving JSON exactly one year ago, to a small Angular front end. It's was a little disheartening to see an empty page loading and then making JSON calls and thinking that by then a server rendered page would have been already fully loaded. That's why I said that it's all or nothing: either a single page app or many html pages. A few single app pages don't do well, unless they are preloaded with server rendered content.
Two output formats is a reasonable solution, but one that you don't have to maintain if you go with a framework (such as Angular) that can render JSON back into HTML. It really boils down to whether you want to support a more complicated server or a more complicated client (or drop support for any clients that aren't yours).